1. Field of the Invention
The present invention relates generally to improvements in Electrical Discharge Machining processes, and more particularly pertains to new and improved graphite electrode constructions for use in such processes and methods of making such electrodes.
2. Description of the Prior Art
Electrical Discharge Machining (EDM) is considered a nontraditional machine process because there is no tool force utilized. The cutting tool is actually an electrode. This tool never touches the workpiece. EDM processes require that both the electrode and the workpiece must be made of materials that conduct electric currents. The electrode is merely a platform from which sparking originates. The shape of the electrode determines the shape of the part being machined. EDM can be analogized to lightning in some aspects. The sparking in an EDM process, unlike lightning, however, is very controlled. The electronic power source controls the spark time on in microseconds and the spark time off in microseconds, as well as the peak power of each spark. During the sparking process, electrons move from the electrode to the workpiece. These electrons represent the force which causes the metal in the workpiece to be removed. When the electrons strike the surface of the workpiece they release their energy in the form of heat. Actually three conditions happen at the point of impact by an electron on the surface of the workpiece. These conditions are: heating of a very small portion of the surface where the electron bombardment impacts the workpiece surface, melting of the workpiece as a result of this bombardment, and ultimately continued heating of the workpiece to the point of surface vaporization. Each spark removes a very small amount of workpiece material.
During the 1960's EDM processes utilized only copper electrodes. These copper electrodes were produced by conventional means such as turning, milling, grinding or drilling. Most of the time, due to the difficulties of machining these materials, the electrode cost was at least equal to the workpiece machining cost. EDM users rapidly came to the conclusion that EDM could progress only with the help of a new technology which allowed an economical production of complex shaped electrodes and followed a set of procedures that can be indefinitely repeated.
Although even today there are many die shops that restrict themselves to the use of copper electrodes, especially for fine stamping dies, graphite has become more popular. Graphite is easily machined by conventional processes, although there are problems due to its fragility. Methods have been developed that yield good quality electrodes. Abrasive methods such as Total Form Machining (TFM) have been used successfully to provide good quality graphite electrodes.
In practice, the metallic electrode in an EDM process can accept a shorter off time or a higher duty cycle than a graphite electrode. As a rule of thumb, the more complex and difficult the electrode, the greater the off time or smaller the duty cycle should be. Although if the electrode is well flushed, then the off time can be reduced considerably. In other words, the duty cycle can be increased. The object in obtaining the optimum duty cycle is to achieve the shortest off time, highest duty cycle, because by reducing the off time and increasing the duty cycle, the metal removal rate increases while the wear on the electrode diminishes.
In most instances the electrode is the cathode and charged negatively, while the workpiece is an anode and charged positively. The use of negative polarity has its advantages, especially with graphite electrodes, because the metal removal rate is double the rate for an electrode graphite that is charged positively. However, the wear on the electrode in this mode is also increased considerably. It is quite possible that the wear in a negative polarity graphite cathode can be up to ten times higher. The negative polarity graphite electrodes find a particular useful application in large molds or forge dies, where the electrodes can be produced economically on an abrading machine, often called TFM. This application does not increase substantially labor-intensive charges for the electrodes and the higher metal removal rate obtained by the negative polarity is a great advantage.
The prior art has used many ways of holding a graphite electrode, for example, a base plate, a shaft or a collet. Metal electrodes, for example, have been held by screwing, soldering or brazing. A more recent technique is to use adhesive instead of brazing. Conductive adhesives have been used for this purpose. These adhesives, known as two-component adhesives, are charged with graphite or silver. The prior art, however, especially when large electrodes are used, recommends the use of a very thick layer of glue, in addition to screwing the electrode to a base plate, if used, in order to ensure positive electrical conductivity.
In spite of considerable advancement in the art of EDM in the areas of forming the graphite electrode by Total Form Machining and in the use of such electrodes, problems still remain. It was common practice, prior to the present invention, when forming the cathode electrode out of a graphite block by TFM to hold the block in place by the clamps available. These clamps engaged the graphite directly through slots in the sides of the graphite block.
This type of holding system caused the graphite to collapse and break from beneath the clamps during the machining process, allowing the electrode to bang against the tooling bars. The net result was damage to both the tooling bars on the machine bed as well as the electrode being formed. If the broken graphite exceeded two inches in height, the entire electrode would have to be scrapped, and at the very least the break would have to be re-formed and the material resurfaced to fit the clamping system. All this caused a loss of time, material and money. The present invention eliminates this problem.
One aspect of the present invention is to provide a base plate for the graphite block which attaches to the electrode, is readily interchangeable therewith, and takes the force of the holding clamps on the tooling bed. Use of these base plates eliminates slippage of the graphite block during the TFM process. The clamping system with the use of the base plate adds an additional 2 inches of usable graphite to the block thickness because this new clamping technique only utilizes 11/2 inches of graphite at the base rather than the 31/2 inches which was required. This additional usable graphite extends the life of the electrode being formed.
Another problem that existed in the prior art, prior to the present invention, was the creation of scrap graphite resulting from the electrode wearing down too close to its base plate. As was noted earlier, a negatively charged cathode exhibits considerable wear during the EDM process. This wear is compensated for by re-forming the cathode by the TFM process. Each time the electrode is re-formed, the graphite cathode drops further down towards its base until the graphite block is worn down to the point where there is no longer sufficient graphite left for the re-forming. At this point, there is still a considerable amount of graphite left.
For example, making a cathode out of a rough graphite block that costs about $800.00 would leave a scrap electrode after its useful life which still had graphite in it worth about $480.00. In the past, the scrap electrodes were simply thrown away. The present invention puts these electrodes back to use by simply adhering a new block of graphite to its base. This is done without the use of screws or electrode straps, in a manner that provides for electrical continuity between the adhered electrode and the new base block. The procedure extends the life of a graphite electrode indefinitely, as long as it is desired to stack the electrode on a new base block.